It is and it isn't. There are some people who know exactly how long and the physics drop in certain games a rocket has allowing them to pull off these kinds of shots. Knew a buddy of mine that could do it himself, with fairly good consistency to call it not luck.

There was/is a pro TF2 player called reptile that was ridiculous with the soldier (i.e. basically solely rocket launcher). I imagine these types of players exist in all online games, alluding to Arinoth's point.